1. Field of the Invention
The present invention relates to an optical pickup device that projects a light beam to an optical recording medium so that information can be read or written and an objective lens actuator that is provided to the optical pickup device. More specifically, the present invention relates to a structure of triaxial driving actuator that is capable of driving an objective lens in three directions.
2. Description of Related Art
Optical recording media including a compact disc (hereinafter referred to as a CD) and a digital versatile disc (hereinafter referred to as a DVD) are widely spread. Furthermore in recent years, for a purpose of increasing quantity of information on the optical recording medium, studies about high density recording on the optical recording medium have been performed. For example, a high density optical recording medium such as HD-DVD that is a high definition DVD and a Blu-Ray Disc (hereinafter referred to as a BD) are being available. When information is recorded or reproduced on such an optical recording medium, an optical pickup device is used, which projects a light beam to the optical recording medium so that information can be recorded or read.
The optical pickup device is provided with an objective lens actuator, so that an objective lens supported by a movable holder can move in a focus direction that is a direction perpendicular to a recording surface of the optical recording medium and a tracking direction that is a radial direction of the optical recording medium. Thus, focus adjustment and tracking adjustment are performed while a spot of the light beam projected to the optical recording medium via the objective lens can follow a recording track of the optical recording medium. Recently, as high density optical recording media have appeared, conditions for a light beam spot to follow the recording track have become strict. In particular, when the optical recording medium generates wobbling while it rotates, it is required to adjust an inclination of the objective lens.
Therefore, as described in JP-A-2005-38527 or JP-A-6-251405 for example, there is developed an objective lens actuator that is a so-called triaxial actuator capable of moving the movable holder not only in the focus direction and the tracking direction but also in a tilting direction that is a direction in which the objective lens is tilted. This triaxial actuator is provided with an objective lens, a fixed portion, and an objective lens holder that retains the objective lens and is supported by the fixed portion via flexible rod-like supporting members so as to be capable of moving in the focus direction, the tracking direction, and the tilting direction.
The movement of the objective lens holder in the focus direction, the tracking direction, and the tilting direction can be realized by electromagnetic actions between each of a focus coil, a tracking coil, and a tilting coil that are provided to the objective lens holder and a magnet disposed at a position opposed to them via the objective lens holder. FIGS. 8A and 8B are diagrams to show a structure of a part of a conventional triaxial actuator having the structure described above. FIG. 8A is a schematic perspective view to show a relationship among the focus coil, the tracking coil, and the tilting coil, and FIG. 8B is a schematic diagram of the objective lens holder when viewed from its side.
As shown in FIGS. 8A and 8B, a focus coil 101 is formed substantially in an elongated rectangular barrel shape and is disposed horizontally. The tracking coils 102 are formed substantially in a rectangular shape and are disposed at the outer peripheral side of the focus coil 101 in a separating manner horizontally at four sides. In addition, the tracking coil 102 is disposed in the direction substantially perpendicular to the focus coil 101. The tilting coils 103 are formed substantially in a rectangular shape and are disposed at the lower side of the focus coil 101 in a separating manner horizontally at the right and the left.
As shown in FIG. 8B, an objective lens holder 104 on which the objective lens 106 is mounted is moved by interaction with a magnet (not shown) in the focus direction that is the vertical direction when the focus coil 101 is supplied with current, in the tracking direction that is the right and left direction when the tracking coil 102 is supplied with current, and in the tilting direction that is a circumferential direction around an axis that passes through a rotation center 105 and is perpendicular to the paper when the tilting coil 103 is supplied with current. Then, by this movement in the three directions, the light beam spot can follow the recording track of the optical recording medium correctly.
However, studying about forces generated by the electromagnetic action in the focus coil 101, the tracking coil 102, and the tilting coil 103 of the objective lens actuator, it is understood that the direction of the force generated by electromagnetic action and the moving direction of the movable holder is the same as to the focus coil 101 and the tracking coil 102, but the direction “a” of the force generated in the tilting coil 103 is different from the moving direction b of the objective lens holder 104 (see FIG. 8B).
Therefore, the objective lens actuator of the conventional structure has a low utilization factor of the force generated in the tilting coil 103 and a low drive sensibility for tilting the objective lens, which means excessive power consumption. In addition, there is another problem caused by an undesired force generated in the tilting coil 103 in the direction other than the tilting direction, which is that the rotation center of the objective lens holder 104 is shifted so that the spot position of the light beam passing through the objective lens cannot follow the recording track of the optical recording medium correctly.